The present invention relates generally to improvements in magnetic data encoding. More particularly, the invention relates to improvements in data encoding density achieved through precise placement and reading of magnetic transitions on a magnetic medium.
Magnetic media such as tape has long been popular for storage of data. Magnetic media is used in many applications, of which some examples are computer hard disks, magnetic tape cartridges suitable for backup of computer hard disks, and magnetic stripes for storage of data on identification cards. As technology advances and information needs become greater and greater, users of all storage media, including magnetic media, find advantages in storing information more and more densely. Magnetic media store information through the establishment of magnetic transitions on a medium. Increasing density through increasing the number of transitions on a medium often leads to increases in cost, which can be substantial if a very large number of transitions are to be placed in a small space. Moreover, because of inherent limitations in technology, as well as limitations inherent in the nature of the medium, there is typically a limit to the information density which can be achieved using a magnetic medium, especially if it is desired to use readily available components. Furthermore, standards which prevail in many applications such as magnetic identification cards specify the density of the information which can be placed on the media through magnetic transitions. For example, standards governing the encoding of information on magnetic stripes of credit cards often specify the information to be placed on the card and the method by which the information is to be written. In order to achieve compatibility between the different cards and card reading devices, the cards must conform to the standards. Many standards allow no room for other information beyond the information specified in the standards, if increasing the information content of the cards is to depend on increasing the number of magnetic transitions on a card.
Similar limitations exist in other magnetic media such as magnetic tapes, in cases in which it would be desirable to increase the density of information stored on the medium, but because of standards to be followed or limitations of the medium it is impossible to increase the number of magnetic transitions on the medium. This is particularly true in cases involving financial identification cards such as credit or debit cards. Magnetic stripe credit or debit cards typically have only a small area available for magnetic storage of information. The content of. this information is specified by standards promulgated by institutions such as MASTERCARD(copyright) and VISA(copyright). The standards typically do not leave room for any additional information.
It would often prove useful to add additional security information to credit or debit cards, but because of the standards for design of the cards, no room is available for adding additional information in the form of additional magnetic transitions.
In addition, situations arise in which it may be necessary to store information on a magnetic medium in a way which is transparent to devices operating according to a particular standard, but which can be read by devices operating according to a different standard. For example, it may be desirable to encode authentication information on an identification card such as a credit card in a way which is transparent to preexisting readers, but which can be read by readers designed to detect the authentication information.
Encoding standards for magnetic data storage typically define a nominal placement of magnetic transitions used to represent data, and allow: for deviations within a certain tolerance from this nominal placement. Magnetic transitions are typically recognized by sensing of peak points in a signal produced by the passage of the magnetic media alongside a read head. In a typical magnetic encoding process, the peak points, and therefore the recognized magnetic transitions, deviate from the nominal. The deviation from the precise placement of the peak points is referred to as xe2x80x9cjitter.xe2x80x9d The Fernandez patent, cited above, describes techniques for reading patterns of jitter in order to define a magnetic signature for a magnetic medium, used for authentication of that magnetic medium.
If deviation from the nominal position of a magnetic transition, or jitter, can be controlled, this represents additional information which can be used to encode data. The Fernandez patent cited above describes the use of already present jitter as a source of additional information for authentication of a medium, but does not describe the use of controlled jitter as a means for encoding information on a medium.
There exists, therefore, a need in the art for a technique for increasing density of information stored on a magnetic medium which does not require an increase in the number of magnetic transitions on the medium and which uses precisely controlled positioning of placement of magnetic transitions on the medium to define additional information that is transparent to devices not equipped to read the additional information.
An information storage system according to the present invention includes a media writer adapted to make precise placements of magnetic transitions on a magnetic medium. The writer writes conventional bits along a magnetic medium by placing magnetic transitions along the length of the medium according to a predefined standard. For example, in order to write a binary xe2x80x9c1xe2x80x9d, the writer may place a transition from low to high, followed after a narrow interval by a transition from high to low, with a narrow interval following before a transition. To write a binary xe2x80x9c0xe2x80x9d, the writer may place a transition from low to high, followed after a wide interval by a transition from high to low, with no interval following before the next bit is able to begin. Placement of each transition is defined by a standard which defines a nominal placement and an allowable deviation from the nominal.
In order to encode additional bits, the writer places transitions between bits in positions deviating from the nominal. To encode an additional xe2x80x9c0xe2x80x9d, the writer places the transition ahead of the nominal position. To encode an additional xe2x80x9c1xe2x80x9d, the writer places the transition behind the nominal position. The deviations from the nominal are within tolerances defined by the standard for encoding bits, which may be a standard used by prior art readers. For example, one prior art standard for writing data on a magnetic identification card allows a deviation of 8% from the nominal. In order to encode an additional xe2x80x9c0xe2x80x9d, the writer may place a transition such that the distance between the transition beginning the bit and the transition ending the bit is at least 3% greater than the nominal distance between the transitions. In order to encode an additional xe2x80x9c1xe2x80x9d, the writer may place a transition such that the distance between the transition beginning the bit and the transition ending the bit is at least 3% less than the nominal distance between the transitions. Requiring that the deviation be at least 3% from the nominal distinguishes deviations which represent additional data from randomly occurring deviations. Conventional writers are typically capable of placing transitions within 3% of the nominal position, so that if a transition is seen to deviate 3% or more from the nominal position, it may be safely interpreted as representing data.
In order to prevent accumulation of errors, additional bits are represented by adjusting placement of transitions between conventionally encoded bits. Adjusting placement of a transition separating members of a pair of bits need not change the placement of the transitions at the beginning and end of the pair of bits. Therefore, the transitions at the beginning and the end of the pair of bits can remain in their nominal positions.
A reader according to the present invention reads transitions in order to decode bits, interpreting sequences of transitions as xe2x80x9c1sxe2x80x9d or xe2x80x9c0sxe2x80x9d according to predetermined standards for encoding and decoding data. The reader also detects placement of transitions which deviate from nominal placement in order to detect additional data. The reader detects deviations of transitions between bits. If the distance between transitions is at least 3% greater than the nominal distance, for example, the deviation from the nominal may be interpreted as representing an additional xe2x80x9c0xe2x80x9d, while if the distance between transitions is at least 3% less than the nominal distance, the deviation from the nominal may be interpreted as representing an additional xe2x80x9c1xe2x80x9d.
A prior art reader which reads a card or other magnetic medium encoded according to the techniques of the present invention can read bits which were conventionally encoded without detecting the additional bits encoded by placement of transitions. This allows, for example, the addition of security or other authentication information which can be detected by appropriate readers but which does not interfere with readers not suitably equipped to detect the information.
If a magnetic medium encoded with a writer according to the present invention is read by a prior art reader, the bits represented by transitions will be read. Bits encoded by placement of transitions will not be detected and will not interfere with the reading of the conventionally encoded information.
A more complete understanding of the present invention, as well as further features and advantages of the invention, will be apparent from the following Detailed Description and the accompanying drawings.